increases the number of larvae that hatch successfully. Defoliators: Populations Rise In the early 1950s, favorable climate for budworm and Fall Statewide development resulted in millions of acres of defoliated western hemlock in Southeast Alaska. Outbreaks of spruce aphid are closely tied to the survival of Rob Progar, Research Entomologist, USDA Forest overwintering adults. Other appear to be Service, Pacific Northwest Research Station; Jim Kruse, genetically predisposed to outbreak in regular cycles : ESSAY Entomologist, Forest Health Protection of 10 to 30+ years (Figure 11).

Defoliating insects eat the leaves or needles of forest trees and are found throughout Alaska on all tree species. Defoliators significantly affect both conifer and deciduous trees in Alaskan ecosystems, and can cause tree mortality with consecutive years of defoliation. In maritime ecosystems dominated by conifers, such as Prince William Sound and Southeast Alaska, defoliating insects tend to be more significant agents of change. If complete defoliation of conifers occurs early in the summer, before buds have been formed for the following year, trees can be badly damaged or even killed.

Figure 11. Aspen defoliators undergo periodic cycles over time. Mean When defoliator populations are epidemic, vast area of infested aspen forest over five-year periods for large aspen acreages can be affected. During an outbreak, nearly tortrix (dark gray), and aspen leaf miner (light gray). Data collected during annual Aerial Detection Surveys. every tree in a stand can be damaged to varying degrees. In addition to the effects on individual The last few summers marked a shift from internal leaf tree physiology, defoliators can also impact wildlife feeders to external leaf feeders as the most common habitat, ungulate forage, aquatic system productivity, sources of damage in Alaskan forests (see Map timber and property values, and the aesthetics of 1 on page 7). In 2012, over 280,000 acres of external forests and recreation areas. Extensive hillsides of feeding damage was observed on Alaskan hardwood brown or red defoliated habitat in the midst of an trees and shrubs, particularly birch and . Unlike outbreak can be quite alarming. Fortunately, the effect many of the leaf miners, which tend to attack only is often ephemeral; the dead leaves or needles drop a single host species or genus, external leaf feeding to the ground and the plants re-foliate later in the insects are often polyphagus, feeding on a wide season or during the following spring. Defoliation variety of hosts. Currently, the most active defoliating can provide a number of ecological benefits: larvae species belong to the families Geometridae and represent an abundant food source for many species Tortricidae. The species responsible for damage vary of birds and other wildlife; increased light penetration regionally. The most destructive geometrids in 2012 to the understory can promote the growth of shrubs were the Bruce spanworm (Operophtera bruceata) in and forbs as browse for ungulates; and leaf litter and Southcentral Alaska, the northern marbled carpet larval scat inputs create a pulse in soil nutrients. moth ( citrata) in Interior Alaska, and the Bruce spanworm as well as the variable girdle moth Defoliator outbreaks tend to be cyclic and closely () in Southeast Alaska. tied to weather conditions. Dramatic increases in defoliator populations require synchrony between In 2012, aerial detection surveys documented larval emergence and tree bud break (food availability). over 177,500 acres of defoliation of birch trees and Weather conditions affect insect development, shrubs (Figures 12 and 13). Approximately half of reproduction and dispersal, as well as host phenology. the defoliation was on shrubby dwarf birch (Betula For example, high temperature during pupation nana, B. glandulosa), and half was on birch trees (B. and egg-laying of western black-headed budworm neoalaskana, B. kenaica). Much of this activity was

Forest Health Conditions in Alaska - 2012 17 observed on the Kenai and Alaska Peninsulas, but Several species of geometrid and sawflies were it was also common throughout Interior Alaska. abundant on in 2012 (Figure 14). The incidence The primary insects found feeding on birch trees of alder defoliation remained high, with about 58,000 and shrubs were the birch leaf roller (Epinotia acres observed, primarily south of the Alaska Range solandriana) and rusty tussock moth (Orgyia antiqua) (Figure 15). The rusty tussock moth caused scattered in Interior Alaska, and a variety of geometrid moths alder defoliation north of the Alaska Range. All of in Southcentral Alaska. The characteristic yellowing these insects cause partial to complete defoliation; it of birch foliage, caused by birch aphids and other is very difficult to distinguish between their feeding piercing-sucking insects, was observed in pockets patterns from the air. scattered around Interior Alaska (10,000 acres total). In 2012, 13,000 acres of damage to aspen caused by external leaf feeders were detected during aerial surveys, most notably the large aspen tortrix. Of 27,000 total acres of cottonwood defoliation, leaf-feeding beetles caused 9,500 acres of damage to cottonwood. defoliation was noted on 26,500 acres, and the rusty tussock moth was the most common insect detected on willow in ground checks.

FHP will continue to monitor, conduct ground checks, and coordinate with landowners to characterize defoliation events and identify the insects involved in 2013. Although most trees and shrubs recover following defoliation events, some are made more susceptible to secondary insects and pathogens or are killed outright. It is important that defoliating insects are identified when outbreaks occur to confirm that damage is caused by insects that are native to Alaska. Most native defoliators have natural agents to keep their populations in check, such as predators, parasitoids, and diseases. Monitoring insect defoliator population dynamics can improve our understanding of insect population response to weather events and climate conditions, allowing us to better Figures 12 and 13. Severe birch and shrub defoliation near Amanka Lake in southwestern Alaska, predict defoliator activity.  close to Wood-Tikchik Sate Park..

18 U.S. Forest Service Alaska Region, State & Private Forestry INSECTS

Figure 14. Trichiosoma triangulum larva found feeding on alder leaves.

Figure 15. Alder and other shrubs have been heavily defoliated in Southcentral and southwestern Alaska in recent years. A variety of different insects have been contributing to the observed damage.

Forest Health Conditions in Alaska - 2012 19